Tan has been involved with the startup community in San Francisco, most recently creating an interactive Google map of startup-centric neighborhoods in San Francisco.

Our own Audrey Watters calls the move "huge for YCombinator", relating how she watched Tan conduct back-to-back mentoring sessions for hours on end at a recent startup event.

Rafael Corrales, CEO of the Silicon Valley VC-backed startup LearnBoost, agrees that the move is a big one.

"It's amazing how Y Combinator has recently recruited some of the best guys in the Valley," said Corrales. "It's like a miniature version of what Facebook has done the past few years."

Posterous itself was a YCombinator-funded startup, so Tan will be returning to his roots in some ways. Tan writes that he feels comfortable that Posterous will be in good hands:

Just as Posterous has prospered, grown and changed, so to is it time for me to evolve my role. Effective today, I'm ending my day-to-day development with Posterous and moving into an advisory role. Though my day-to-day may change, my faith in the team and the product is unchanged and unwavering. Posterous is in good hands and on the right track to fulfilling its potential. I am proud of what we've built together and look forward to the future with anticipation to see where the team and you, the users, take this very special community. [...] I am greatly thankful to our team, investors and most of all our users for all the amazing work and adventures. Thanks for all of your support.

Good luck Garry!